You're going to get dirty, so bring clothes to change into, e.g. swimsuit, sandals, hat and jacket. You have to reserve the tour in advance. They pick you up from your place, take you to the camp and then take you back to your place. We made the reservation on the website.
It's a real close experience with the elephants. Our 3-year-old nugget was a bit scared, so he just watched them from afar. But, for our 5-year-old nugget, it was amazing!
We did the half-day experience, but there's also a full-day option. For us, a half day was enough as the route to get there is really bumpy and wore out our nuggets. The time of the day will depend on at what time the tour will pick you up.
Avoid rainy season, from May till October, but actually, that will depend on the time of the year you're in Chiang Mai. So, if it's raining, you just get wet, that's all.